According to a new paper from three researchers at Harvard and MIT, removal of a strict High-Occupancy Vehicle (HOV) policy had negative effects on traffic in Jakarta, Indonesia, the world’s second largest metropolitan area. Congestion increased even on streets with no restrictions and during times when the policy had not been in place. The decision to remove the policy provided an opportunity for a natural experiment in congestion mitigation.

Starting in 1992, the city enforced a three-in-one policy (three occupants for every one vehicle) for a 12-lane artery, Jalan Sudirman, providing access to Jakarta’s Central Business District. Like many large roadways, this road is congested during peak commute times, and the HOV policy was aimed at reducing the number of cars on the road during these times.

However, a workaround sprung up, with professional passenger “jockeys” and even mothers with children standing next to the road offering to serve as passengers for a small fee. In response to crime and safety concerns the authorities unexpectedly announced in April 2016 that the policy would be lifted with only seven days’ notice, providing an opportunity to observe the impacts to traffic before and after the restrictions were lifted.

Although the easing of the requirement was initially a trial, it was made permanent one month later. The authors collected Google Maps traffic data and included a series of counterfactual examples and control roads (ones that had never had an HOV policy) to test their theory that HOV lanes can mitigate congestion even on roads that are not restricted. Their analysis proved the hypothesis. When the high-occupancy policy was abandoned, the traffic did indeed become much worse, but not just on the principal arterial.

After the policy was abruptly abandoned in April 2016, delays rose from 2.1 to 3.1 minutes per kilometer (min/km) in the morning peak and from 2.8 to 5.3 min/km in the evening peak. The lifting of the policy led to worse traffic throughout the city, even on roads that had never been restricted or at times when restrictions had never been in place. In short, we find that HOV policies can greatly improve traffic conditions.

The authors theorized several possible explanations. The easiest explanation was that commuters were now not only driving to work during peak hour, but also used their cars for other trips, creating more congestion outside the previous HOV policy times and locations. Another possibility is that the increased traffic on the principal road created hypercongestion that both decreased the capacity of the arterial and also forced traffic onto surrounding streets and clogged feeder roads with people trying to access the arterial.

Within the study’s parameters the authors make a case for reintroducing HOV policies in cities or communities that experience unruly traffic or demoralizing congestion, and demonstrate that HOV policies can impact congestion even on roads without the restrictions.